Abstract

Recent findings from Voltage Sensitive Dye Imaging (VSDI) experiments done on anesthetized cats (Arieli et al., 1995;Arieli et al., 1996;Kenet et al., 2003;Tsodyks et al., 1999;Grinvald et al., 1991) indicated that the amplitude of ongoing activity (primarily synaptic potentials) is large, suggesting that it may play an important role in cortical processing by ‘shaping’ the evoked activity and behavior. VSDI was recently implemented also on the awake monkey (Seidemann et al., 2002;Slovin et al., 2002) allowing monitoring of activity from the same patch of cortex for up to a year. Several questions can be addressed now: what are the spatial-temporal characterizations of the ongoing activity in early visual areas of the behaving monkey? Does it affect the evoked activity? How is it related to the functional architecture? We investigated the cortical activity in the primary visual cortex of a behaving monkey during both evoked and ongoing conditions. We combined simultaneous VSDI with electrophysiological recordings of local field potential (LFP) single and multi unit activities. In the evoked condition, the monkey was trained to fixate for 10s while presented with a full field moving grating, whereas, during the ongoing condition, the monkey was required to sit quietly in a dark room. We found that the VSD signals in both conditions are often highly similar to the LFP, just like in the anesthetized cat. The similarity between the VSD signals and LFP was highest within the α (9-14 Hz) frequency band. Although the ratio between amplitude of ongoing and evoked activity was much smaller (~1/6) than what was found in the anesthetized cats, extensive spike triggered averaging (STA) of the VSD signals revealed coherent spontaneous activity also in the awake primate. These results suggest that ongoing activity may play an important functional role in the awake primate rather than being an epi-phenomenon of anesthetized preparations.